Support tmpfs backed sandbox mount for LCOW containers

[jiechen0826]

This PR adds the support for tmpfs backed sandbox mount. The tmpfs mounts are mounted inside UVM under the directory at /run/gcs/c/<SANDBOX-ID>/sandboxTmpfsMounts.

Containers can utilize a tmpfs-backed sandbox mount using sandbox-tmp:// prefix as follows,

"mounts": [
      {
          "host_path": "sandbox-tmp://s1",
          "container_path": "/sndbox/m1"
      }
]

If checking from the container's perspective,

.\crictl.exe exec -i $CONTAINER cat /proc/self/mountinfo
129 122 0:41 /s1 /sndbox/m1 rw,relatime - tmpfs tmpfs rw